1. Field of the Invention
The present invention relates to a soundproof sheet for vehicles, a manufacturing method thereof, and a dash silencer for vehicles using the soundproof sheet.
2. Description of the Related Art
Conventionally, in this type of a soundproof sheet for a vehicle, for example, a dash insulator for a vehicle disclosed in Japanese Patent Application Laid-Open No. 2009-12561 was proposed. The dash insulator is constructed with a sound absorbing material installed in a dash panel at an inner side of a vehicle compartment thereof and with a sound insulation member installed on the sound absorbing material at an instrument-panel lower side thereof.
In the dash insulator having the above-described construction, the sound absorbing material absorbs mainly noises of a high frequency range in noises from an engine room. The sound insulation material insulates mainly noises of a low frequency range in noises of the engine room under its lamination structure with the sound absorbing material.
Accordingly, the dash insulator functions so as to perform soundproofing of noises in the engine room from the interior of the vehicle compartment over high to low frequency ranges of the noises in the engine room. Thus, noises which cannot be absorbed or insulated by the dash insulator is transmitted into the interior of the vehicle compartment.
By the way, according to the dash insulator as constructed above, when the noise is transmitted from the above-described dash insulator to the inner side of the vehicle compartment, the noises are transmitted from a portion of the sound absorbing material corresponding to the instrument panel to the inner portion of the instrument panel and is transmitted to the interior of the vehicle compartment from a portion of the sound absorbing material corresponding to the lower side of the instrument panel and the sound insulation material.
Herein, as described above, the portion of the dash insulator opposing the inner portion of the instrument panel is not the sound insulation material but a portion of the sound absorbing material opposing the instrument panel.
Therefore, if the noises transmitted to the inner portion of the instrument panel as described above are reflected by the instrument panel and is absorbed by the opposing portion of the sound absorbing material, phenomenon that the reflected noises are confined within the instrument panel are reduced.
However, the sound absorbing material is merely a single layer. Therefore, it is difficult to effectively absorb noises from the engine room by the sound absorbing material. This means that the sound absorption performance of the dash insulator to noises from the engine room is insufficient.
In other words, in the dash insulator, even if the above-described sound insulation material is installed so as not to oppose the inner portion of the instrument panel, as described above, the sound absorption performance of the sound absorbing material to the reflected noises in the above-described instrument panel is insufficient, and the sound insulation performance of the sound insulation material is also insufficient under lamination structure of the sound insulation material with the sound absorbing material.
On the contrary, like a laminated sound absorbing layer in a soundproofing material for a vehicle disclosed in Japanese Patent Application Laid-Open No. 2009-18746, it may be considered to construct the above-mentioned sound absorbing material by a lamination structure of a base sound absorbing layer and a foamed resin sheet which have different surface densities to each other.
However, in such a simple lamination structure, the sound absorption performance or the sound insulation performance, as described above is still insufficient.